In the related art, there is known an ultrasonic sensor including a semiconductor substrate having an opening portion, two layers of electrodes on an insulating film layer formed on the surface of the semiconductor substrate by blocking the opening portion, and a piezoelectric element formed with a PZT ceramics thin layer interposed between the two layers of electrodes (see JP-A-2010-164331).
The efficiency of transmission and reception of the ultrasonic sensor depends on the deformation distribution in the ultrasonic sensor, but if it is desired to cause the deformation in the film thickness direction to be significant, a two-dimensional shape when the ultrasonic sensor is viewed in the film thickness direction may be caused to have a low aspect ratio.
Examples of a structure of the ultrasonic sensor include a structure in which transmission and reception are performed on an opening portion side, and structure in which transmission and reception are performed on an opposite side of an opening portion. In all structures, even if only a shape (shape viewed in film thickness direction, that is, shape in a planar view, and hereinafter, referred to as a “shape”) of a piezoelectric element is set to have a low aspect ratio, deformation in the film thickness direction does not become significant. That is, an opening portion and an active portion of a piezoelectric element provided thereon are required to be the same size and shapes having low aspect ratios. However, if the shape of the opening portion is caused to be the same size as the active portion of the piezoelectric element, partitions forming the opening portion inhibit propagation of ultrasonic waves, an efficiency decreases or a size of the opening portion becomes excessively small so that workability becomes worse.